1. Field of the Invention
The present invention relates to a pulse tube refrigerating system which serves for generating very low temperatures and in particular to an efficiency-improved system of such a type.
2. Discussion of Background
As is well-known, a conventional pulse tube refrigerating system is constructed such that a pressure wave generator, a regenerator, a cold head and a pulse tube are connected in series in this order. Between the pressure wave generator and the pulse tube, there is formed a closed operating space which is filled with a working fluid such as helium gas. When the pressure wave generator is turned on, an alternating mass flow of the working fluid is caused, which results in an establishment of a phase difference between pressure oscillation and displacement of the working fluid. This leads to that in the regenerator a heat flow is generated from a cold head to the pressure wave generator and the cold head is cooled down to a very low temperature.
In order to obtain the maximum cooling ability or heat transfer ability at the cold head, it is well known that setting the phase difference at about 90 degrees is effective. This fact can be known from a thesis, for example, reported in Advances in Cryogenic Engineering, Vol. 35, P1191/1990). On the basis of this, an improved pulse tube refrigerating system has been proposed in a report (Proceedings of the fifth International Cryocooler conference P127/1988). In the improved system, a phase shifter is employed in order to establish a suitable phase difference of about 90 degrees between pressure oscillation and displacement of the working fluid.
As shown in FIG. 3, a conventional pulse tube refrigerating system 103, which is equivalent to the foregoing improved pulse tube refrigerating system, includes a pressure wave generator 1, a regenerator 2, cold head 3, and a pulse tube 4 which are connected in series in this order. The pulse tube has a high temperature end 44 which is in connection with an expansion piston system 6. The expansion piston system 6 includes a cylinder 61, a piston 62 reciprocally fitted in the cylinder 61, a spring 58, a wire coil 59 wound around the cylinder 61, and a resistor (not shown) connected to the wire coil 59 in series. In the cylinder 61, there are formed a space 63 and a space 64 at opposite ends of the piston 62. The spring 58 serves for supporting the piston 62 and is connected at its upper end to the cylinder 61 and at its lower end to a back end of the piston 62. A clearance seal is constructed between an axial outer surface of the piston 62 and an inner end wall of the cylinder 61.
The piston 62 is in the form of a permanent magnet and establishes an electromagnetic induction system by cooperating with the coil 59. The expansion piston system 6 has an eigenfrequency which depends on a mass of the piston 62, a spring constant of the spring 58 and a gas spring constant of the space 64. Induction currents created while the piston 62 is reciprocated are supplied to the resistor and an amount of heat is generated at the resistor. The resulting heat is rejected or radiated, as Joule heat, to the surroundings. Due to such a heat generation, a damping force is applied to the piston 62. In addition, a compulsory force is applied to piston 62, whose magnitude is a multiplication of a cross-section area of the piston 62 and a pressure difference between the space 63 and 64. Thus, the expansion piston system 6 is constituted as a damped and compulsory force system.
In the expansion piston system 6, if the eigenfrequency thereof is in coincidence with the operating frequency of the pressure wave generator 1, when the piston 62 under movement away from the high temperature end 44 of the pulse tube 4 passes through the center point of the oscillation of the piston 62, the speed of the piston 62 takes its maximum value and the absolute value of current induced at the wire coil 59 becomes maximum. Thus, the compulsory force applied to the piston 62 becomes maximum which is obtained by subtracting the pressure in the space 64 from the pressure in the space 63. This means that the maximum setting efficiency of the system 6 is established by setting a phase difference of 90 degrees between the maximum pressure in the space 63 and the farthest position of the piston 62 relative to the high temperature end 44 of the pulse tube 4. On the other hand, due to the volume of the pulse tube 4, at a low temperature end 43 of the pulse tube 4 the phase difference at the low temperature end 43 of the pulse tube 4 is less than the foregoing phase difference of 90 degrees by tens of degrees.
In light of this, in the expansion piston system 6, the eigenfrequency thereof is set to be less than the operating oscillation frequency of the pressure wave generator 1 in order to maximize the compulsory force applied to the expansion piston 62, which is defined by the subtraction of the pressure in the space 64 from the pressure in the space 63, before the piston 62 passes through the center of oscillation thereof This means that a phase difference of above 90 degrees is set between a time when the pressure in the space 63 becomes maximum and a subsequent time when the piston 62 takes the farthest position relative to the high temperature end 44 of the pulse tube 4. This establishes a substantial 90 degrees in phase difference at the low temperature end 43 of the pulse tube 4, resulting in an improvement of producing very low temperature or cold at the cold head 3.
However, in the foregoing pulse tube refrigeration system 103, an amount of expansion work performed by the expansion piston system 6 is rejected to the surrounding as a heat, which brings that an amount of compression work to be done at the pressure wave generator 1 becomes large and an efficiency of producing very low temperature is not better than expected. The following is an analysis of such a phenomena. Approximately, the work Wexp which is rejected to the surroundings from the system 6 can be represented as the following equation. EQU Wexp=.pi.A P.sub.o .xi..sub.o sin .THETA.
where A is a cross-section area of the expansion piston 62,
P.sub.o is an amplitude of the pressure oscillation in the space 63, PA1 .xi..sub.o is an amplitude of the displacement of the expansion piston 62, PA1 .THETA. is the phase difference between the maximum pressure in the space 63 and the farthest position of the expansion piston 62 relative to the high temperature end 44 of the pulse tube 4 when the piston 62 is under movement away from the high temperature end 44 of the pulse tube 4. PA1 TH is a radiating temperature at the pressure wave generator 1, PA1 TC is a temperature produced at the cold head 3. PA1 A ratio of Wexp/Wcomp is given as follows. EQU Wexp/Wcomp=(1-Wp/Wcomp)TH/TC.
On the other hand, the Wcomp which is done by the pressure wave generator 1 can be represented as the following formula or equation. EQU Wcomp=Wp+(TH/TC)Wexp
where Wp is a work loss which is done as a result of pressure drop at the regenerator 2,
As an example, substituting 0.2, 80K, and 320K for Wp/Wcomp, TH, and TC, respectively, the ratio of Wexp/Wcomp becomes 0.2. This means that 20 percent of the work performed at the pressure wave generator 1 is rejected at the expansion piston system 6 to the surroundings as a heat. In addition, if the value of TC is approximately the same as the value of TH, the ratio of Wexp/Wcomp approaches 1, which indicates that most of the work performed at the pressure wave generator 1 is rejected at the expansion piston system 6 to the surroundings as a heat.